1. Field of the Invention
The present invention relates to signal processing device and method, and particularly to signal processing device and method that allow accurate detection of a subcarrier with a simple configuration.
2. Description of the Related Art
In recent years, a contactless IC card system employing a contactless IC card is becoming prevalent in a transportation system, a security system, an electronic money system, etc.
In such a system employing a contactless IC card, upon the entering of the contactless IC card into a communication-allowed distance from a reader/writer, initially the reader/writer radiates electromagnetic waves to the contactless IC card via an antenna. In this state, the reader/writer transmits a signal for requesting data reply to the contactless IC card via the antenna. In response to this signal, the contactless IC card load-modulates the requested data as reply data and sends out the load-modulated signal to the reader/writer via an antenna part.
The reader/writer receives the signal load-modulated by the contactless IC card and demodulates the signal to thereby acquire the reply data. Such demodulation processing is generally realized by a demodulator incorporated into the reader/writer.
The demodulator is configured as e.g. a circuit incorporated into an IC chip. As described above, it is possible that the demodulator is incorporated into a reader/writer and demodulates a signal transmitted from a contactless IC card. In addition, it is also possible that the demodulator is incorporated into a contactless IC card and demodulates a signal transmitted from a reader/writer.
Hereinafter, as the operating mode of demodulation processing by the demodulator, the operating mode when a signal transmitted from a reader/writer is received will be referred to as the card mode, and the operating mode when a signal transmitted from a contactless IC card is received will be referred to as the reader/writer mode. In the following, the demodulator that operates in the reader/writer mode will be mainly described.
Furthermore, in the description of the present specification, the one-data period representing one-bit data in a demodulated signal will be referred to as the etu (Elementary Time Unit), and it is assumed that the data value of each bit is modulated by the bit coding system defined for each communication system.
Presently, the format of a signal transmitted from a reader/writer to a contactless IC card or the format of a signal transmitted from a contactless IC card to a reader/writer is defined by the standards of e.g. ISO14443 and ISO18092. Three types exist as the types of the signal format defined by these standards.
Specifically, the Type A system (ISO14443-A), the Type B system (ISO14443-B), and the Type C system (referred to also as the Felica system) exist.
For example, a contactless IC card of the Type A system load-modulates a carrier (carrier wave) of 13.56 MHz by a subcarrier of 847 KHz (to be exact, 847.5 KHz) depending on reply data to a reader/writer, to thereby generate a reply signal. Specifically, in the Type A system, in 1 etu representing one-bit data, a data value “1” is represented by a symbol in which the subcarrier is superimposed during the first-half period of this 1 etu for example. Furthermore, in 1 etu representing one-bit data, a data value “0” is represented by a symbol in which the subcarrier is superimposed during the second-half period of this 1 etu.
Moreover, the above-described three Types have frame header information and communication end information different from each other. For example, in the Type A system, at the communication start, a symbol of series D comes as the first-bit received signal (start of communication (SOC)). This serves as the frame header information. Furthermore, in the Type A system, the advent of a symbol of series F (without subcarrier modulation) indicates the communication end.
For example, Japanese Patent Laid-open No. 2006-33281 (hereinafter, Patent Document 1) discloses a related-art reader/writer device that receives a signal load-modulated by the subcarrier in a contactless IC card of the Type A system.
According to Patent Document 1, the reader/writer repeatedly transmits a signal for requesting data reply to the contactless IC card of the Type A system (this will be referred to as the polling processing).
If the contactless IC card of the Type A system exists near the reader/writer, the reader/writer receives a reply sent out from this contactless IC card. At this time, the reader/writer extracts a subcarrier component from the received signal. Furthermore, the in-phase component (I-signal) and the quadrature component (Q-signal) of the subcarrier component are detected to be supplied to a demodulator.
The demodulator squares the I-signal and the Q-signal to add the squaring result and take the square root of the addition result. Then the demodulator supplies a calculation result signal obtained as the result to a moving average unit. In the calculation result signal, the signal level of the subcarrier component superimposed on the received signal appears.
The calculation result signal is integrated by the moving average unit in units of ½ etu, and the obtained integral result signals are sequentially supplied to a shift register.
A timing generator takes timing synchronization in units of ½ etu with the received signal and generates an internal clock rising at the end timing of the ½-etu cycle to output the internal clock to the shift register and a subcarrier signal level detector. Furthermore, the timing generator takes timing synchronization in units of 1 etu with the received signal and generates an internal clock rising at the end timing of the etu cycle to output the internal clock to the subcarrier signal level detector.
The timing synchronization in units of 1 etu and in units of ½ etu between the internal clock and the received signal can be taken by detecting the SOC. Specifically, the SOC detected from the received frame is regarded as the start point and an enable is generated at each of the cycles of 1 etu and ½ etu defined by the standards.
The shift register sequentially latches the results of the moving average of the ½-etu interval in synchronization with the input clock of ½ etu. Thereby, the shift register stores the integral results each corresponding to a respective one of first-half ½ etu and second-half ½ etu of the unit symbol interval.
A subcarrier signal determiner reads out, from the shift register, the integral results each corresponding to a respective one of first-half ½ etu and second-half ½ etu of the unit symbol interval as the subcarrier component signal level of each ½ etu, in synchronization with the clock of 1 etu. Subsequently, a threshold determination is made twice every 1 etu for the obtained subcarrier component signal level of each ½ etu and thereby whether a subcarrier component is present or absent is determined.
In the threshold determination, if the signal level of the subcarrier component in a ½-etu period surpasses a predetermined threshold, it is determined that the subcarrier exists in this ½-etu period. If the integral result signal indicating the signal level of the subcarrier component in the ½-etu period is equal to or lower than the predetermined threshold, it is determined that the subcarrier does not exist in this ½-etu period.
In the threshold determination, if it is determined that the subcarrier exists in the first-half period and the subcarrier does not exist in the second-half period, it is determined that this one-data period is equivalent to series D (data value “1”). If it is determined that the subcarrier does not exist in the first-half period and the subcarrier exists in the second-half period, it is determined that this one-data period is equivalent to series E (data value “0”).
If it is determined that the subcarrier exists in neither the first-half period nor the second-half period, it is determined that this one-data period is equivalent to series F (e.g. non-modulation period). If it is determined that the subcarrier exists in both the first-half period and the second-half period, it is determined that this one-data period is equivalent to collision.
Due to the above-described scheme, the demodulator can reproduce data returned from the contactless IC card of the Type A system one bit by one bit and also can make determinations relating to the frame end and collision.